Aerial cameras with photosensitive electronic, mostly high resolution surface sensors have been known for a long time and are used for taking aerial images that are, for example, captured from a flying object moving at a specific air speed at a specific flight altitude. Known aerial cameras in this case typically have a central, global diaphragm shutter.
Such an aerial camera is described, for example, in publication document DE 199 32 065 A1.
In order to sharpen imaging, the photosensitive pixels (5) of the surface sensor which, in particular, comprises a multiplicity of CCD or CMOS rows, are brought into register with the image plane of the objective. Only small positional deviations between the image plane and the focal plane—that is to say the surface of the surface sensor—are permitted, depending on the focal length and the aperture ratio of the objective used.
It is known from sport photography that for the purpose of sharply imaging a runner, for example, the camera has to be panned during the exposure time in accordance with the movement of the runner What is obtained is a blurred image of the runner that has a so called motion blur.
The same problem also occurs with opto electronic reconnaissance systems that are used from aircraft. During the exposure time or integration time for, for example, a specific map square of the earth's surface that lies in the capture range of the aerial camera, a displacement of the map square to be taken occurs with reference to the aerial camera arranged in the flying object. This effect likewise then leads in the case of such photographs to fading and/or motion blur of the aerial images taken.
U.S. Pat. No. 5,460,341 discloses an aerial camera, it being possible to adjust the focal plane in various directions in order to compensate the focal drift, doing so by means of linear actuators. The aerial camera comprises a focal drift sensor that senses the position of the parts forming the imaging optics, and a temperature sensor that senses the temperature of various areas of the imaging optics. The computer simulation of specific values for the focal drift is then used to track the focal plane as a function of the values of the focal drift sensor and of the temperature sensor.
DE 195 02 045 discloses a focal plane with CCD rows of an aerial camera that can be moved perpendicular to the optical axis by means of a piezoelectric actuating element. The aim thereby is to compensate instances of image fading and/or motion blur that are caused during the exposure by the forward flight movement, and thus by the relative movement of the camera in relation to the terrain captured.
EP 1 570 314 B1 and U.S. Pat. No. 3,744,387 also describe aerial cameras that compensate forward flight movement by means of actuating elements that effect an appropriate tracking of the sensor and/or of the film.
Other known approaches to reducing image fading in the photographing of objects moving relative to the camera are directed towards shortening the exposure times.
Thus, it is known that in the case of the use of a slit diaphragm shutter, for example, it is possible to achieve shorter exposure times—in comparison with required exposure times for a central, global diaphragm shutter.
In the case of a slit diaphragm shutter, use is made of two diaphragm elements that are also denoted as curtains. In the basic position, in which no exposure is performed, a first of the two curtains is closed so that it completely covers the sensor, whereas the second curtain is open and does not cover the sensor. For the purpose of taking a picture, the first curtain is opened and moves, usually at constant speed, over the image. As soon as the desired exposure time is reached for the sensor line first exposed by the opening of the first curtain, the second curtain begins to close and to move over the image at, for example, a constant speed—and in the same direction as the first curtain. A slit diaphragm thus rolls or runs over the image, as it were.
The consequence of this type of diaphragm shutter is that individual locations of the surface sensor (that is to say the sensor lines that are respectively exposed and covered again one after another for exposure purposes) are exposed at different points in time. The respective sensor lines therefore exhibit different mean exposure times.
Consequently, it is possible—in comparison with the duration of exposure for central diaphragms—for the duration of exposure of the individual sensor lines to be greatly shortened. It is true that image fading and/or motion blur can thereby be reduced in photographs of objects moving relative to the camera. However, since in the case of aerial photographs the projected image is moved relative to the sensor and relative, in particular parallel, to the curtain slit diaphragm by the forward flight movement in the case of aerial photographs, the effect of the different exposure times of the individual sensor lines is that the aerial image of the terrain as sensed by the sensor is either compressed, that is to say squashed, or stretched, that is to say drawn out in length, depending on whether the slit diaphragm is displaced in the same or opposite direction as the projected image over the surface sensor.
WO 2007/08159 describes a method of photography—for example for sport photography—where use is made of a slit diaphragm shutter, the displacement of a moving object being determined from two photographs captured successively. A corrected image is formed as a function of this displacement and of the different exposure times of the individual sensor rows—this being done by appropriately modifying one of the two photographs captured.
However, it happens in the case of aerial photography that the drift rate of the projected image does not remain constant relative to the sensor during photographing, since the drift rate depends, inter alia, on the air speed and the flight altitude above the terrain to be captured. This can lead to undesired distortions in the aerial photographs.